Spectrograph aboard the GAIA satellite operates in the near-IR, in
the 8490–8740 Å window accessible also from the ground.
The most important parameter yet to be determined is the spectral resolution.
Realistic estimates of the zodiacal light background are obtained and a total
of correlation runs are used to study the accuracy of
radial velocity measured by the spectrograph as a function of resolution,
magnitude of the target, its spectral type and luminosity class.
Accuracy better than 2 km s-1
is achievable for bright stars if a high enough dispersion is chosen.
Radial velocity error of 5 km s-1 is at for Cepheids and
at 17.7 for horizontal branch stars. Even for very
faint objects, with spectra dominated by background and readout noise,
the optimal dispersion is still in the 0.25/0.75 Å/pix range. This
is also true for complicated cases such as spectroscopic binaries or if
information other than radial velocity, i.e. abundances of individual
elements or stellar rotation velocity, is sought after.
The results can be scaled to assess performance of future ground based
instruments.

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